Polyester has long been recognized as a desirable material for textile applications including garments, upholstery and numerous other uses. The processes for manufacture of polyester are relatively well known and straight forward to those knowledgeable in the art, and fibers made from polyester can be appropriately woven or knitted to form textile fabrics. Polyester fibers can be blended with other fibers such as wool or cotton to produce fabrics which have the enhanced strength, durability and memory aspects of polyester and retaining many of the desired qualities of the natural fiber with which the polyester is blended.
As with any fiber, the particular polyester fiber from which any given fabric is formed must have properties suitable for end use of the fabric. In many applications such as sleepwear fabrics, draperies, and bedspreads, for example, it is desirable the textile fabric have the property of flame resistance. Flame resistant fabrics are defined as fabrics that will self extinguish when an ignition source is removed. Requirements are set forth in various tests including the NFPA 701-1977.
One technique for attaining the flame resistance of fabrics of polyester fiber is to modify the polyester with carboxyphosphinic acids. Such modified polyester fibers and their use in fabrics are disclosed in U.S. Pat. Nos. 3,941,752; 4,033,936; and RE 30,783. In fact, these patents describe flame resistant linear polyesters made from modified polyesters consisting of dicarboxylic acid components and diol components and phosphorus containing chain members being structurally units of the formula ##STR1## which constitute about 1-20 mole percent of the acid component of the polyester. In this formula R is the saturated open-chained or cyclic alkylene, arylene, or aralkylene having from 1-15, preferably from 2-10 carbon atoms and R is an alkyl radical having up to 6 carbon atoms or an aryl radical or an arakyl radical as described in U.S. Pat. No. 3,941,752. Such modified polyester fibers are currently available from Hoechst Celanese Corporation.
Although fabrics containing polyester fibers as described above provide flame resistance, such synthetic fibers have an undesirable propensity upon prolonged use to exhibit small, compact groupings or entangled fibers (i.e., fuzzballs) on the fabric surface. Such fiber groupings commonly are termed "pills" and tend to form and to tenaciously adhere to the surface of the fabric as the fabric encounters surface abrasion during normal use. The aesthetic appearance of fabric accordingly may be adversely influenced by these relatively small groupings of entangled fibers which are retained on the surface of the fabric.
Heretofore, it has been believed that such pills can be traced to the relatively high strength of the synthetic fibers present in the fabric. For instance, the pills may be more or less permanently attached to the fabric surface by one or more synthetic polymer fibers extending out of the fabric which will resist breakage as the surface abrasion continues. This theory of pill formation is supported by the significant lower level of the retention of undesired fuzzballs o the surface of the fabric consisting solely of cotton fibers following the same surface abrasion conditions. It is believed that the entangled cotton fibers which form at the surface of the fabric readily break away since the cotton fibers are of an inherently lower strength.
This pilling problem may be observed in fabrics formed in whole or in part from polyethylene terephthalate (PET) fibers. Pills commonly are observed on the fabric formed from blends of cotton and PET staple fibers following use in service and during the cleaning process including laundering or dry cleaning. While the pills may be observed in fabrics having a wide variety of construction, they are more commonly observed on loosely constructed fabrics, particularly knitted fabrics.
One approach heretofore proposed to reduce the pilling of fabrics is to reduce the tenacity (or strength) of the PET fibers by using a low molecular weight (measured as intrinsic viscosity) PET polymer. Low intrinsic viscosity provides a general indication of reduced polymeric chain length and leads to fibers having a lesser strength. Accordingly, when such entangled fibers become free on the surface of the fabric following abrasion, the fibers tend to cleanly break away and not hold a fuzzball at the surface of the fabric.
It has been found, however, that when this approach is followed, it is more difficult initially to form and process the PET into a fabric using standard processing conditions because of the reduced tenacity. Such reduced tenacity can lead to a premature fiber breakage and processing disadvantages. These disadvantages have been overcome by gentler textile processing conditions. However, it has also been known that intrinsic viscosity reduction does not resolve the problem of low pilling to the satisfaction desired in the market.
It is also well known to chain branch polyester polymers in order to reduce pilling in the fabric. Chain branching of polyester fibers can be accomplished by inclusion of chain branching agents such as tetrafunctional chain branching agents, in particular, pentaerythritol and tetraethyl orthosilicate.
U.S. Pat. No. 3,576,773 discloses low pilling PET fibers containing trifunctional or tetrafunctional branching agents. Pentaerythritol is listed as a branching agent and is shown used in Example 5. U.S. Pat. No. 3,335,211 discloses low pilling PET fibers made in the presence of a polycondensation catalyst of antimony or titanium and adding an oxysilicon compound prior to melt spinning such as tetraethyl orthosilicate. Although it is recognized that trifunctional and tetrafunctional branching agents may be used to overcome pilling of PET fibers, inclusion of such branching agents in flame resistant polyester fibers at normal processing intrinsic viscosities of polyester has been found not to achieve the desired low pilling.
There remains a need to develop a processable polyester fiber having flame resistance and low pilling properties while also maintaining the other properties desired in the resulting fabric, especially the aesthetic properties of the fabric resulting from the polyester fiber.